This invention relates generally to artificial gas lifting systems used to produce hydrocarbons from well bores and more specifically to a subsurface valve assembly and method for alternately injecting and venting gas for producing heavy oil.
Once a well bore has been drilled between the earth's surface and a producing formation, oil will flow naturally to the surface if the reservoir pressure is great enough to overcome the pull of gravity upon the column of fluid in the well. If the well does not flow naturally, an artificial lifting system of some sort must be employed. Various pumping systems known in the art have been used for flowing hydrocarbons to the surface and include electrical and hydraulic downhole pumps and sucker rod pumps.
In recent years, the so-called "gas lift" systems have gained increasing popularity for producing oil. The gas lift systems can be generally classified as continuous flow and intermittent flow. In the continuous flow system, gas passes continuously into the fluid column through a gas lift valve downhole, thereby aerating the fluid column, making it weigh less. When the static head of the fluid column is reduced enough, pressure from the reservoir overcomes the resistance of the fluid column and the well flows.
In the intermittent system, reservoir fluid is permitted to rise in the tubing for a set interval of time without gas injection. Gas is then injected very rapidly through a gas lift valve. The large influx of gas pushes a portion of the fluid column to the surface. Gas pressure is then decreased to allow fluid to again rise in the tubing.
A variation of the intermittent gas lift system is the chamber lift pump developed by Johnnie Elfarr of Palestine, Tex., and marketed through the Thermo Pump Company of Palestine, Tex. The Elfarr chamber lift pump uses high pressure gas to overcome the weight of the fluid column above the pump in the well bore to pneumatically lift the produced fluid to the surface by a cyclic process of gas injection followed by a hold and then an exhaust sequence. The Elfarr system has several advantages over traditional pump systems including low initial cost, lack of mechanical complexity, and ease of maintenance. Also, typical of gas lift type systems, is the feature that such systems are not adversely affected by deviation of the well bore. Such systems are also less susceptible to problems caused by production of sand, paraffin, salt, or scale.
In spite of these advantages, the Elfarr system is designed so that all of the injection gas is vented back up the injection gas conduit. The use of a single conduit for injection and venting has several drawbacks. These disadvantages include the possibility of back pressure buildup being applied to the formation or the production chamber where oil accumulates, reduced drawdown in the well bore pressure at the formation, longer operating cycle time, and higher gas volume requirements per cycle.